The present invention concerns a liquid crystal electro-optical display device including spacers and a method for manufacturing such a device.
The invention concerns the industrial sector of manufacturing liquid crystal visual display devices, formed of two close plates or substrates made of glass, quartz, silicon or other material, between which is formed a sealed enclosure in which liquid crystals are confined.
These liquid crystal display devices include in particular so-called alignment layers deposited on the opposing faces of the two substrates, and which are used to align and orient the liquid crystal molecules. The conditioning of the surface of these alignment layers is very important, since the orientation of the molecules and, consequently, the display contrast, depends upon it.
A first known technique for forming the alignment layers consists in depositing alignment materials such as SiO2, MgF2, WO3, MoO3 or others along a determined angle of incidence over the surface of the substrates. In order to deposit these alignment materials over the surface of the substrates along the desired angle of incidence, a vaporising source is deposited with the inclination selected with respect to a straight line perpendicular to the surface of the substrates. Among suitable deposition methods, one may cite vacuum deposition, the atomisation method and other known vapour phase deposition methods.
Another known technique for forming the alignment layers consists in applying, for example by centrifuging, an alignment material such as polyimide solution, to the surface of the substrates. Non polymerised polyimide solution remains non polymerised after evaporation by drying the solvent. In a subsequent manufacturing step, the liquid crystal display cell is heated, for example for one hour at approximately 250xc2x0 C., which causes polymerisation of the polyimide alignment layers. After polymerisation, the alignment layers are rubbed.
Liquid crystal display cells of the type described above also include spacers intended to maintain a constant distance or separation between the two plates of the cells and to give the cells satisfactory mechanical rigidity. In methods known to date, maintenance of the distance between the two plates is generally assured by balls or discontinuous fibres of perfectly controlled geometrical dimensions, distributed over one of the plates and bonded before the second plate is set in place. Since this initial technique has certain drawbacks, such as, in particular, that imprecise positioning of the balls can lead, locally, to mechanical deformation of the cells, replacing these balls with continuous spacers, formed of wires or bars of square, circular, polygonal or other cross-section has been proposed. These wires or bars, for example made of glass or quartz, are fixed by adhesion, bonding or sealing either onto only one of the two plates, their height being equal to the desired spacing, or on each of the plates so as to criss-cross, the series of spacers assuring the spacing between the plates and giving the resulting cell the desired mechanical rigidity.
These spacers, whether they are balls, cut fibres, continuous bars or others, have nonetheless the drawback of disturbing the alignment of the liquid crystal molecules, which prevents the polarisation rotation. Different problems then arise the most noteworthy of which are as follows:
formation of disturbed zones causing a reduction in contrast, or generating the  less than  less than reverse rotation greater than  greater than  phenomenon which affects the uniformity of the display;
loss of alignment after a long period of operation;
formation of halos around activated segments resulting from a too low a rotation angle of the liquid crystal molecules.
In order to overcome these problems, Japanese Patent No. 10003082 proposes making spacer balls in the form of fine particles of plastic material obtained via polymerisation of a fluorite type monomer emulsion, known for its liquid crystal molecule alignment properties. However, the polymer balls resulting from this method have an amorphous structure, so that the liquid crystal molecules tend to align themselves radially around the balls. Thus, even if, in proximity to these balls, the alignment of the liquid crystal molecules is more ordered, this alignment nonetheless still differs substantially from the homogenous or homeotropic planar alignment of the liquid crystal in the rest of the volume of the display cell. Even at one degree less, the problems of reduction in contrast and non uniformity therefore subsist.
The object of the present invention is thus to overcome the above drawbacks and others by providing spacers which do not disturb the alignment of the liquid crystal molecules.
The invention therefore concerns a liquid crystal display cell including two close substrates, separated by a sealing frame which delimits a sealed enclosure in which the liquid crystals are confined, and spacers intended to maintain a constant spacing between said substrates, this cell being characterised in that the sealing frame and/or the spacers are formed of an anisotropic polymer which is used to align and orient the liquid crystal molecules.
As a result of this feature, the principal axes of the polymer molecules are aligned along a privileged direction, such alignment being transferred by surface effect to the liquid crystal molecules. The liquid crystal molecules, in turn, will, as a function of the orientation of the polymer molecules, be aligned in a homogenous or homeotropic planar manner at right angles to the spacers, in the same way as in the rest of the display cell volume. The invention thus allows the phenomena of alignment disturbance of the liquid crystal molecules which usually occurs in proximity to conventional spacers or sealing frames to be avoided. A more reliable display is thus obtained, with better contrast and without halos or parasite display points.
The present invention also concerns a method for manufacturing a display cell of the aforementioned type, characterised in that it includes the steps of:
coating a substrate with a monomer layer,
depositing on the monomer layer a mask having openings whose contour corresponds to the shape of the desired sealing frame or spacers,
irradiating the monomer layer through the mask openings by means of a polarised beam, and
removing the mask and the non exposed monomer.